1. Field of the Invention
The invention relates generally to the field of measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole. More particularly, the invention presents method and apparatus adapted for using in pipe assisted wireline and similar applications, including through the drill bit applications.
2. Background Art
Various prior approaches have been suggested and implemented for measuring nuclear magnetic resonance (NMR) properties of earth formations surrounding a borehole to evaluate the earth formations. Most recent generation of NMR well logging instruments employs a static magnetic field produced by a permanent magnet to align nuclear spin magnetic moments of protons or other nuclei present in the earth formations. The aligned spin magnetic moment is typically brought into excited state by applying an RF magnetic field. RF voltages are induced in the receive antenna as a result of precessional rotation of nuclear spin axes of hydrogen protons about the static magnetic field with characteristic resonance or Larmor frequency corresponding to the static magnetic field strength.
Practical wireline NMR downhole tools are represented by U.S. Pat. No. 4,717,878 issued to Taicher et al. representing a centralized type tool design, U.S. Pat. No. 5,055,787 issued to Kleinberg et al. representing a skid type side looking design with quasi-homogeneous static magnetic field, and U.S. Pat. No. 6,452,388 issued to Reiderman, et al. representing side-looking gradient type design. A permanent magnet used in all the practical wireline NMR tools generates polarizing magnetic field that aligns nuclear spin magnetic moment. The angle between the nuclear magnetization and the polarizing magnetic field is then changed by applying a pulsed radio-frequency (RF) magnetic field at a frequency corresponding to the static magnetic field magnitude at a predetermined distance from the NMR tool. A sequence of RF pulses can be designed to manipulate the nuclear magnetization in order to acquire NMR relaxation property of the earth formation. For the NMR well logging the most common sequence is the CPMG sequence that comprises one excitation pulse and a plurality of refocusing pulses. One of the main challenges of the NMR downhole measurements is to achieve an acceptable signal-to-noise ratio (SNR). Typical SNR for downhole NMR measurements is 3-10 per one measurement cycle. For the configurations of the magnets and the antennas of the NMR tools represented in the US patents '878 '787 and '388 the SNR is less for smaller tool diameter.
Pipe assisted wireline method of acquiring borehole data gives unique capability of acquiring formation data in difficult well situations, as well as in high angle or horizontal wells. This method typically require small diameter logging tools capable of being lowered or raised through the drill string. Typical tool diameter for this application is about 2 inches. There are no NMR logging tools having outer diameter small enough to be used in the pipe assisted wireline application. A prior art NMR logging tool of this diameter would have an unacceptably small SNR. Therefore there is a need for an ultra slim NMR logging tool with sufficiently high SNR.
Thus known in the art instruments do not give any satisfactory solution for an ultra-slim NMR logging tool with sufficiently high SNR. Therefore it is an objective of the present invention to provide a solution for high SNR slim NMR tool suitable for the pipe assisted wireline application.